The present invention relates to a template or plate conveyor for use in a thermoforming press and more particularly for receiving and moving mouldings of objects thermoformed in the thermoforming press throughout a plurality of working or treatment stations, while keeping the thermoformed objects in the mutual position in which they were obtained in the mould of the thermoforming press.
It has already been suggested in the art of thermoforming objects to use a modular template or plate conveyor for carrying thermoformed objects through one or more stations at which the objects are worked or treated. Such conveyors comprise a pair of chains carried and wound on two pairs of spaced co-axial sprocket wheels which delimit two parallel (going and return) lengths or runs at least one of which is provided with driving means for moving the conveyor forwards, and a plurality of templates or plates extending across the two chains and kept at a predetermined distance or pitch from one another by the same chains. The distance between the various plates is determined, as it is clearly understood, by the dimensions (length) of the modular plates, by the total length of the conveyor and by the mutual distance of the working or treatment stations at which the plates and thus the objects must stop. One such conveyor is disclosed in U.S. Pat. No. 5,591,463.
The plate or template conveyors used up to now have a number of disadvantages that, besides making their use complicate, limit to a significant extent the efficiency of the whole thermoforming system.
The mere fact of providing a necessarily fixed pitch between the various templates carried by the chains makes it necessary to adapt the positioning and the distance between the various working or treatment stations to such a pitch, which is not easy task nor is it always permissible since it would be necessary to have wide overall spaces (especially spaces extending in length) available for the conveyor and one must take into account that other components of the thermoforming system occupy part of the available space.
Moreover, it is necessary to use an excessive and never optimum number of templates since they must stop at waiting, and thus quite often idle, zones in order to comply with the need of a constant pitch between the templates. For the same reason the templates or plates must all effect movements equal in length and at the same speed owing to their constraint with the carrying chains.
Thus, with passing of the time positioning inaccuracies and parallelism inaccuracies of the various templates occur at the work (working or treatment) stations owing to irregular elongation of the chains and unavoidable differences in the thermal expansion-shrinkage and traction between the two chains, inaccuracies that quite often get worse as a result of repeated adjustments of the chain tension, wear and friction between the chain links and the slide guides of the chains.
The use of chains also causes noise which increases with prolonged use owing to irregular elongations and wear that are not compensated by the chain links, which results in bad mashing between the chains and the pairs of sprocket wheels that support and drive the chains, low transmission efficiency and consequent waste of driving energy.
Moreover, the return runs of the chains at the pairs of the sprocket wheels for supporting and driving the chains instability problems arise which makes sometime difficult to keep the objects in their respective receiving seat in the transport template, especially when dealing with very low objects with relatively large dimensions, owing to the fact that components of inertial forces crop up which act on the object, e.g. in the sense of lifting and ousting them from their seats in the template along the descending length of the conveyor.
Finally, it is almost always difficult to provide work stations between the forward and backward runs of the chains, also because the distance between the chains depends upon the diameter the sprocket wheels, which cannot exceed certain limits otherwise the conveyor would be exceedingly cumbersome.
The main object of the present invention is to provide a template conveyor for a thermoforming press, in which templates move in sequence along a closed or endless path without the use of restraining chains between the templates, but independently from each another, i.e. with the possibility of making movements that differ both in length and speed to eliminate or drastically reduce the above mentioned drawbacks in the thermoforming presses provided with conventional conveyors.
Another object of the present invention is to provide a template conveyor for a thermoforming press, which ensures a high degree of accuracy both in the stopping and parallelism positioning between the templates or in the attitude of the templates even after long intervals of work without the need of conducting periodical settings.
Not least object of the present invention is that of providing a template conveyor for a thermoforming press which makes it possible to obtain a high production in the time unit, high transmission efficiency, and thus a low driving energy consumption and high reliability.
In accordance with one preferred aspect of the present invention, there is provided a plane-template conveyor for a thermoforming plant for obtaining a plurality or moulding of thermoformed hollow objects including at least one thermoforming press having at least one male die or mould, at least one female die and at least one picking up and transferring group for sequentially picking up and transferring moulds of thermoformed objects from a die to a template conveyor, the said conveyor including a support frame which delimits an endless path and a plurality of templates movably mounted on the said endless path and each having a number of receiving seats equal to that of the objects of one mould, characterized in that it comprises a plurality of driving means spaced along the said endless path to cause a sequential and discontinuous movement of the said templates, but at a predetermined speed between one template and the other.
Advantageously, the said path comprises a first and a second straight guide paths and an end connecting path at each end of the said first and second straight paths, along which the said templates are kept in a constant parallelism attitude.
Moreover, the said driving means advantageously comprises a rotatable cam having a helicoidal variable-pitch profile extending along the said first and second straight paths for sliding engagement with at least one template along the said straight paths, and transfer means for transferring the said template from the said first path to the said second path.